The suspension systems of automobiles, trucks, motorcycles and many other vehicles customarily have passive hydraulic "shock absorber" or damper assemblies connected in parallel with springs between the vehicle frame and axle components to minimize motion of the vehicle frames and to thereby enhance the riding comfort, stability and/or safety of the vehicles. The output forces of the passive dampers are usually entirely dependent upon the rate of relative movement between the interconnected vehicle components and upon the fixed, or sometimes manually adjustable, fluid flow characteristics of the dampers. A serious deficiency of a damper of the aforesaid type is that it will at times amplify, rather than attenuate, motion of the vehicle frame. This highly undesirable result will occur whenever the vehicle frame and axle components are moving in the same direction (e.g, upwardly or downwardly) and the absolute velocity of the axle component is greater than that of the frame component.
In contrast to a passive hydraulic damper, an active hydraulic actuator connected by suitable control means to a hydraulic pump or similar source of high pressure hydraulic fluid can and does produce output forces totally independent of relative movements between the masses interconnected by the actuator. When associated with suitable means for rapidly and appropriately controlling the flow of pressurized hydraulic fluid to it, a fast-acting hydraulic actuator can therefore produce optimal motion attenuation of one of the masses in substantially all situations. However, the size, weight and/or expense of a hydraulic pump or the like and the necessary associated control components makes the use of an active actuator impractical in the vehicle suspensions of some vehicles (e.g., motorcycles) and renders the use thereof unattractive in many other types of vehicles.
An alternative to a motion-attenuation system employing an active actuator, and also to a system employing an uncontrolled passive damper, is a "semi-active" system along the lines of that disclosed in U.S. Pat. No. 3,807,678. The system of such patent does not employ an active actuator, and therefore does not need or include a hydraulic pump or similar source of high pressure fluid, but achieves a degree of motion attenuation closely approaching that realizable by a fully active system. This desirable result is achieved by control of the output of a purely passive damper so as to cause, among other things, such output to be of substantially zero magnitude at all times when motion amplification rather than motion attenuation would ensue from a damper output of any appreciable magnitude.
While affording many benefits, a semi-active damper control system intended for general vehicle-suspension use must satisfy a variety of diverse requirements if it is to be commercially acceptable. It must be exceedingly rapidly acting and relatively inexpensive. It should be readily adaptable to and mountable in association with the existing suspension and frame components of different types of vehicles. This necessitates, among other things, that the system components be of lightweight compact construction and that they not be orientation-dependent; i.e., the operation thereof should not be adversely affected by the particular orientation in which they are mounted or by subsequent changes in such orientation due to movement of the vehicle structure to which they were secured. The system components should be highly reliable in operation and therefore should be well-protected against damage from the harsh environmental conditions (exposure to dirt, grime, water, snow, ice, impacts from rocks and the like, extreme changes in temperature, etc.) to which vehicle suspensions are customarily subjected. At the same time, however, the system component should be readily accessible for purposes of adjustment, inspection and/or repair if required.